Method for balancing a turbofan engine or other rotating system

ABSTRACT

A method for balancing rotating machinery, such as gas turbine engines, to minimize vibrations. The method involves operation of the engine for a period of time at varying power levels and ranges of other operational parameters representative of the system operating envelope to obtain vibration data (amplitude and phase) for the full range of dynamic responses of interest. This usually includes time at elevated power settings until the engine reaches thermal stability, altitude variation, etc. as well as the full engine operating range. The full set of vibration data measured during the engine run is analyzed to generate unique unbalance states. The unique unbalance states are then analyzed and the mean unbalance state is identified. Balancing masses can then be installed or removed in accordance with a balance solution that is equal and opposite to the mean unbalance state.

RELATED PATENT APPLICATION

This application is a continuation of and claims priority from U.S.patent application Ser. No. 13/466,333 filed on May 8, 2012.

BACKGROUND

This disclosure generally relates to systems and methods for balancingrotating machinery to reduce or minimize vibrations. In particular, thedisclosed embodiments relate to systems and methods for balancing gasturbine engines.

It is either impossible or nearly impossible, as a practical matter, tobuild a rotating structure that is perfectly balanced upon manufacture.Any such structure will produce a certain amount of undesired vibrationto a greater or lesser extent. Such vibration is usually passed throughmounts that restrain the rotating part of the structure, and cantherefore manifest itself as unwanted noise or vibration in adjacentstructures. As is known to those skilled in the art, synchronousvibration may be characterized by an amplitude (i.e., magnitude) and aphase angle (i.e., direction). Thus, the vibration of a part may berepresented as a vector or phasor.

One type of rotating machinery susceptible to undesired vibration is thehigh-bypass turbofan engine used in commercial aviation. Such engineshave a large number of rotating elements. These rotating elements can begrouped according to the relative speed of rotation. Some of therotating elements form a low-speed rotating system and other rotatingelements form one or more high-speed rotating systems. Morespecifically, each rotating system of a gas turbine engine comprises anupstream rotating multi-stage compressor connected to a downstreammulti-stage turbine by means of a shaft. The low-pressure turbine andlow-pressure compressor are connected by a low-pressure shaft; thehigh-pressure turbine and high-pressure compressor are connected by ahigh-pressure shaft which surrounds a portion of the low-pressure shaft,with the high-pressure compressor and turbine being disposed between thelow-pressure compressor and turbine. The fan of the turbofan engine isthe first stage of the low-pressure compressor. Vibration caused byunbalances in the various stages of a turbofan engine contributes towear and fatigue in engine components and surrounding structures, andunwanted noise in the passenger cabin of the airplane.

One way of reducing structurally transmitted vibrations is to balancethe rotating systems of aircraft engines on a regular basis. Enginebalancing is well known in the aircraft art. The manufacturers ofturbofan engines have developed techniques for controlling the magnitudeof unwanted vibration by affixing balancing mass to the engine.Typically, only the fan and the last stage of the low-pressure turbineof a turbofan engine are accessible for applying balancing mass afterthe engine is manufactured or assembled. Internal stages areinaccessible as a practical matter.

A known method for applying balancing mass involves the selection of acombination of balancing screws from a set of screws of differentstandard mass, with screws being threadably inserted into respectivethreaded holes located around an outer periphery of an internal turbofanengine component (such as a fan spinner). For example, to achieve abalance, one or more screws of the same mass or different masses can bescrewed into respective threaded holes, thereby producing a center ofgravity which is closer to the axis of rotation than was the casewithout balancing. The total effect of multiple attached balancingmasses can be determined by treating each mass and its respectivelocation as a vector, originating at the axis of rotation, andperforming a vector sum.

Although the unbalances of accessible stages of a turbofan engine arethe primary contributors to engine vibration, the unbalances that oftenreside at inaccessible engine stages also contribute to overall enginevibration. When corrective masses can only be placed on the twoaccessible stages, it is difficult to select masses of the propermagnitude and angular position such that they not only function toreduce vibration caused by specific unbalances at the fan and last stageof the low-pressure turbine, but also reduce the influence of unbalancesat the remaining/other stages of the low-pressure compressor andturbine, as well as over the operating envelope of the engine.

The specification of the location and amount of mass to be applied to arotating system in order to balance it is referred to herein as thebalance solution for the rotating system. In order to determine balancesolutions for rotating systems of turbofan engines, vibration data isobtained. Vibration data is a measure of the amount of vibration that anengine is producing at various locations as the engine is operated atvarious speeds and through ranges of other parameters. Vibration datacan be gathered at an engine balancing facility located on the ground orduring flight. If accelerometers are used to capture rotating systemvibration response, synchronous vibration data may be derived using akeyphasor index on the rotating system. While multiple methods known tothe art can be used to capture and derive vibration data, that data mustcontain a displacement as well as a phase corresponding to synchronousvibration. After vibration data is obtained, the vibration data is usedto derive a balance solution that attempts to minimize the vibration ofthe engine producing the data.

In a known procedure for gathering engine vibration data in flight,so-called “stable” vibration data is captured using the last stablepoint for each of six speed ranges. Sufficient stability is establishedby monitoring amplitude over a period of time and verifying that theamplitude variation is within an acceptable predefined limit. (Stabilitycan also be determined from phase and N1 speed remaining within a givenrange for a given time.) The corresponding shaft speed is also capturedby recording outputs from a tachometer or other shaft speed sensor. Arespective “stable” vibration data point is captured during the flight(vibration amplitude and phase) for each range of shaft speed. Ifstability is achieved, the new vibration data point is substituted forthe present “stable” vibration data point in the corresponding shaftspeed range. After the aircraft has landed, the “stable” vibration datapoints (vibration amplitude and phase) recorded during the flight forthe six shaft speed ranges are extracted and then a least squaresamplitude solution is calculated using the six speed range points andgeneral in-flight influence coefficients. (The method of least squaresis a standard approach to the approximate solution of overdeterminedsystems, i.e., sets of equations in which there are more equations thanunknowns. “Least squares” means that the overall solution minimizes thesum of the squares of the errors made in the results of every singleequation.) The output of this analysis process is adopted as theproposed balance solution. This method intrinsically assumes that thereis a unique relationship between engine speed and vibration response;for example, a particular engine speed always produces a similarvibration response.

Dynamic unbalance characteristics cause sub-optimal balance solutionsusing state-of-the-art balancing techniques. Existing techniques do notaccount for dynamic characteristics related to parameters other thanshaft speed, resulting in solutions that require multiple attempts toobtain acceptable balance states. There is a need for improved methodsof balancing turbofan engines having dynamic unbalance characteristicsto minimize vibrations.

SUMMARY

The subject matter of this disclosure is a method for balancing rotatingmachinery (such as turbofan engines) having dynamic unbalancecharacteristics to minimize vibrations. The method involves operation ofthe rotating machinery through a representative range of operatingconditions that the balance solution is to be effective over. In thecase of an aircraft turbofan engine, this typically includes flying theaircraft on which the engine of interest is installed. Therepresentative range of operating conditions may include operation forextended periods of time over a broad range of shaft speeds, powersettings, and ambient conditions. An example of a set of aircraft flightregimes that may be suitable for satisfying the representative range ofoperating conditions may include aircraft taxi, engine run-up, take-offclimb, level-out, cruise, descent, idle descent and landing. As theengine of interest is operated through the representative operationalenvelope, vibration sensor output is sampled and correlated with akeyphasor signal to produce vibration data. The vibration data may beexpressed as an amplitude having units of displacement and a phasehaving angular units. Influence coefficients may be applied to thevibration data for the purpose of determining an unbalance state. Anunbalance state consists of an unbalance magnitude having units ofmass-length and an unbalance angle. Unique unbalance states areidentified using predefined criteria. For illustrative purposes, anunbalance state could be considered unique if when plotted as a point ona polar plot, no other unbalance state exists within a predefinedboundary. If a second unbalance state is plotted and falls within thepredefined boundary, it would be considered a non-unique unbalance stateand discarded. The unique unbalance states may then be used to determinea single mean unbalance state. The engine is then balanced by adding oneor more balancing masses summing to a mass-length equivalent of the meanunbalance state at a phase angle 180 degrees from the angle associatedwith the mean unbalance state. The method improves the balance of therotating machinery since the balance solution is based on a plurality ofunique data points for each speed or speed range of interest and enginevibration responses throughout the representative range of operatingconditions.

In accordance with one embodiment, a method for balancing a rotatingsystem is provided comprising: (a) attaching a vibration sensor to astructure that vibrates during operation of the rotating system; (b)operating the rotating system for a period of time within an operatingenvelope; (c) converting output from the vibration sensor into vibrationdata points during operation, said vibration data points comprisingamplitude and phase data; (d) calculating respective unbalance statesfrom said vibration data points; (e) identifying unbalance statescalculated in step (d) which differ from each other by at least athreshold range; (f) calculating a mean unbalance state using unbalancestates identified in step (e); and (g) attaching and/or removing one ormore balancing masses to the rotating system to achieve a state which atleast partly compensates for an unbalance represented by said meanunbalance state.

The foregoing method can be used to balance a gas turbine engine on anaircraft, in which case the operating envelope may comprise varyingoperating conditions which influence engine vibration response, such aspower setting, flight regime, altitude and temperature. The method forbalancing a turbine engine may further comprise: pre-storing a set ofinfluence coefficients that are characteristic of a model of the enginebeing balanced; and deriving influence coefficients from that set ofinfluence coefficients, in which case the unbalance states are a vectorproduct based on the respective vibration data points and the derivedinfluence coefficients. The derived influence coefficients in thisexample may relate at least in part to a shaft speed of the engine atthe time when the respective vibration data point was acquired.

In accordance with another embodiment, a method for locating an end of amean unbalance state vector to be applied to a rotating system isprovided, which method comprises: (a) attaching a vibration sensor to astructure that vibrates during operation of the rotating system; (b)operating the rotating system for a period of time within an operatingenvelope; (c) converting output from the vibration sensor into vibrationdata points during operation, the vibration data points comprisingamplitude and phase data; (d) calculating first and second coordinatesof unbalance states for respective vibration data points; (e)identifying unbalance states which differ from each other by more than apreset threshold; (f) calculating a mean value of the first coordinatesof the unbalance states identified in step (e); (g) calculating a meanvalue of the second coordinates of the unbalance states identified instep (e); and (h) locating an end of a mean unbalance state vector usingthe mean values of said first and second coordinates. This method mayfurther comprise: (i) plotting unbalance states identified in step (e)in accordance with their respective first and second coordinates; and(j) displaying a graphical symbol on the plot made in step (i), aportion of the graphical symbol being located at the location of the endof the mean unbalance state vector.

In accordance with a further aspect, a vibration analyzer is providedfor analyzing vibration data points representing amplitude and phase ofvibrations produced by a rotating system having a shaft. In oneimplementation, the analyzer comprises a computer system programmed toperform the following operations: (a) calculating respective unbalancestates for the vibration data points; (b) identifying unbalance statescalculated in step (a) which differ from each other by greater than apreset threshold; and (c) calculating a mean unbalance state usingunbalance states identified in operation (b). The analyzer may furthercomprise memory storing a set of influence coefficients that arecharacteristic of a model of the engine, wherein the computer system isfurther programmed to derive influence coefficients from that set ofinfluence coefficients.

In accordance with another aspect, a system is provided onboard anaircraft for processing data from one or more vibration sensors thatdetect vibrations produced by a turbine engine during flight. In oneimplementation, this system comprises a computer system programmed toperform the following operations: (a) converting output from thevibration sensor into vibration data points during operation, thevibration data points comprising amplitude and phase data; (b)calculating respective unbalance states for the vibration data points;(c) identifying unbalance states calculated in step (b) which differfrom each other by greater than a preset threshold; and (d) calculatinga mean unbalance state using unbalance states identified in step (c).

Other aspects of the balancing method are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will be hereinafter described with reference to thedrawings.

FIG. 1 is a diagram showing an isometric view of one example of anaircraft having gas turbine engines.

FIG. 2 is a diagram showing an isometric view of idealized rotatingcomponents of a turbojet engine.

FIG. 3 is a vector diagram representing vibrations generated duringrotation of the rotating components depicted in idealized form in FIG.2.

FIG. 4 is a flowchart showing steps of a process for analyzing collectedvibration data in accordance with one embodiment.

FIG. 5 is a plot showing unbalance states for a plurality of uniquevibration data points recorded during flight and also showing a meanunbalance state (represented by X) derived from the plotted uniqueunbalance states.

FIG. 6 is a flowchart showing steps of a process for balancing rotatingmachinery, such as gas turbine engines, having transient dynamicunbalance characteristics to minimize vibrations.

FIG. 7 is a diagram showing unbalance states for a plurality of uniquevibration data points recorded during flight, a mean unbalance statederived from the plotted unique unbalance states, and an equal andopposite balance solution to counter the mean unbalance state.

FIG. 8 is a block diagram showing components of a system for monitoringand analyzing engine vibrations during flight and then downloading thebalance solution to maintenance personnel in accordance with a furtherembodiment.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

Various embodiments of methods for balancing gas turbine engines havingdynamic unbalance characteristics will be described in this section. Aperson skilled in the art will appreciate that various steps of thebelow-disclosed methods also have application in procedures forbalancing other types of rotating machinery (such as steam turbines,power trains, gas compressors, etc.).

FIG. 1 shows an aircraft 10 having a pair of gas turbine engines 12.Although the engines may be turbojet engines or turbofan engines, FIG. 2is an idealized depiction of some rotating components of a turbojetengine. In particular, FIG. 2 shows a compressor disk 14, a shaft 16,and a turbine disk 18, both disks being mounted on the shaft 16. This isjust one example of a type of rotating machinery which can be balancedusing the methodology disclosed hereinafter.

Due to many factors, such as manufacturing and assembly tolerances,distortion over time, and/or wear, it is unlikely that the center ofmass of the compressor disk 14 and/or turbine disk 18 will perfectlymatch a geometric axis of rotation 20 of the assembly. Therefore, anattachment point 22 may be included on compressor disk 14 and/or anattachment point 24 may be included on turbine disk 18. Balancing massesmay be added at the attachment points to balance the assembly about itsaxis of rotation 20. A balancing mass is intended to alter the center ofmass of the rotating assembly to better align or coincide with the axisof rotation 20, thereby reducing if not minimizing vibrations.

FIG. 3 is a vector diagram depicting the forces exerted on a gas turbineengine due to vibrations of the compressor and turbine disks. The shaft16 is shown rotating at a speed ω_(n) between a forward unbalance plane26 and an aft unbalance plane 28. The aft unbalance plane 28 correspondsto the location where a turbine disk is located (toward the aft end ofthe engine) and where the vibration induced by the turbine disk acts onthe engine. For a particular instant in time, an unbalanced condition Tof the turbine disk is depicted as a vector 32. Likewise, the forwardunbalance plane 26 corresponds to the location where a compressor disk(or a fan) is located (toward the forward end of the engine) and wherethe vibration induced by the compressor disk (or fan) acts on theengine. For the same particular instant in time, an unbalanced conditionC of the compressor disk is depicted as a vector 30.

The vector 30 has a magnitude and a phase angle α_(n) at the rotationalspeed ω_(n). Similarly, the unbalanced displacement vector 32 has amagnitude and a phase angle β_(n) at the rotational speed ω_(n). Themagnitudes and phase angles of vectors 30 and 32 can be determined usingany suitable technique known in the art, for example, an influencecoefficient method of balancing can be employed. The use of influencecoefficients to balance aircraft engines is well known.

In the case of a high-bypass turbofan engine, a known balancing methodtakes into account the effects of plane unbalances at the fan and thelast stage of the low-pressure turbine, and other plane unbalancescaused by stages that lack means for mounting balancing weights. Theas-built vibration at any location in an engine is at least in part dueto such unbalances, although some stages typically affect the overallunbalance more than others. The influences of unbalances in all of thestages of the rotating system can be related to the accessible stagesusing influence coefficients, which can be derived from the vibrationalresponses when different balancing mass are installed and the system isoperated. These vibrational responses are measured at various shaftspeeds using sensor pickups (e.g., accelerometers). In practice, anyengine that has unacceptably high unbalances after manufacture can befirst run on the ground to measure its as-built vibration via sensorpickups. These measurements can be taken over the engine operating rangeof speeds (measured in rpms). Such data may be recorded when the engineis operating at a specific shaft speed. Measurements while the engine ison the ground do not take into account certain kinds of dynamic responsenow identified in aircraft engines. Such dynamic responses may beobtained from vibration data collected during a variety of conditions.In the case of aircraft engines, the majority of their service life willbe spent in the air, rendering data taken only from test cell or groundconditions of limited value for achieving adequate balancing.

Influence coefficients determined for a particular location arerepresentative of a response at a balancing plane or plane of interest.Influence coefficients may be expressed as a magnitude and a phase shifthaving units of mass-length per displacement and angle respectively. Oneset of magnitude units typically used is gram-centimeters per mil. Oneknown method of calculating influence coefficients for such planes orstages is to use measured data from a representative baseline engineground run, and two trial runs, where data from each trial run isobtained from sensor pickups after placing trial masses on one or bothbalancing planes (e.g., fan and last stage of the low-pressure turbine).Trials may be conducted for the engine operating envelope. Thus, theinfluence coefficients can be calculated since the actual correctivemasses added during any trial run are known, and the resultant change invibration is also known because it can be measured at the sensorpickups.

In theory, the influence coefficients for a given engine, or from oneengine to another of the same type or model, should be nearly identical.However, due to nonlinearity factors, manufacturing tolerances,measurement errors, wear, distortion over time, and other factors, ithas been found that a single set of influence coefficients cannot berelied on. It is known in the prior art to implement a balancing methodby a simple averaging of influence coefficients calculated for a numberof engines, to obtain so-called generic influence coefficients usablefor most engines with some level of confidence, or more exact influencecoefficients can be calculated for each engine in the above-describedmanner. In accordance with a known balancing procedure, a history of anengine's characteristics, including predetermined influence coefficientsfor trim balancing, can be updated and stored for reference whenperforming the balancing procedure. These influence coefficients relatevibration data amplitude and phase at specific locations and shaftspeeds to the mass unbalance at a reference position.

As previously noted, in order to determine balance solutions for therotating systems of aircraft engines, vibration data is obtained. In thebalancing methods disclosed herein, vibration data is gathered duringflight and, optionally, from non-flight conditions. In one embodiment ofan improved balancing method, the engine is operated for a period oftime under different flight regimes such as take-off, climb, level-off,cruise, descent, idle descent, etc. Operation in the different flightregimes exposes the engine to different power settings, altitudes,ambient temperatures, and so forth. Such varying operating parametersinfluence the engine vibration characteristics, resulting in variationwhich can be measured for the full range of dynamic responses ofinterest by acquiring vibration data. The operational envelope of anengine may include time at elevated power settings, thermal loading,altitude variation, changes to rotational inertia as well as the fullengine operating range for different flight regimes. For example,vibration data may be acquired for each engine throughout an entireflight of the aircraft, including takeoff, climb, cruise, descent andlanding. The improved method subjects the engine to different operatingconditions. For a particular speed of interest, there is typically adifferent vibration or accelerometer response associated with eachoperating condition. Multiple responses or data points are acquired foreach speed or speed range of interest. The multitude of responses ismore representative of in-service use or subsequent flights. Themultitude of responses can be used advantageously to determine and applya balance solution. The applied balance solution may have advantagesover balance solutions that are based on single response, from a singleoperating condition, for each speed or speed range of interest.

Various steps of an improved balancing method are shown in FIG. 4.During flight, samples of vibration data are acquired from eachvibration sensor (i.e., accelerometer) (see step 102). A data recorder,such as a digital flight data recorder (DFDR) or an airborne vibrationmonitor (AVM) box, receives vibration data samples (amplitude and phase)from each vibration sensor in the sensor set along with shaft speed datafrom a tachometer. The sensor outputs can take the form of variablevoltage or other signals, which can be converted to amplitude and phaseby the AVM box (or flight data recorder).

After vibration data has been collected for the operational envelope ofthe engines, the data inside the AVM box or DFDR can be processed by avibration data analyzer located onboard the aircraft (optionally, nearlyin real-time) or on the ground. The vibration data analyzer comprises aprocessor that is programmed to select data corresponding to so-calledunique vibration data points for each vibration sensor for eachaccessible plane of imbalance of an engine to be balanced. Theanalytical process depicted in FIG. 4 is for a plane of unbalance of anengine to be balanced. This process involves calculating respectiveunbalance states for vibration data samples 102 acquired by any sensorwithin a range of the plane of unbalance of interest and then extractingthe calculation results corresponding to “unique” unbalance states,i.e., those unbalance states which are more than a specified distancefrom previously derived unbalance states. This process can be duplicatedfor each accessible plane of unbalance of the engine to be balanced.

Still referring to FIG. 4, the first step 104 in the analytical processis to calculate a respective unbalance state for each vibration datasample 102 output by a particular vibration sensor, using selectedgeneral influence coefficients for the particular model of aircraftengine to be balanced. In the present context, which particularinfluence coefficients are selected for use will, among other factors,be a function of the engine shaft speed and which plane of unbalanceneeds to be balanced. For example, for a particular engine model,respective influence coefficients can be provided corresponding torespective shaft speed ranges. For each vibration data point 102acquired while the engine shaft speed was in a particular speed range,the respective influence coefficients will be those associated with thatparticular speed range or an interpolation of such influencecoefficients. Influence coefficients may be expressed in vector form. Anunbalance state may be determined based on a vector product of avibration data point and a respective influence coefficient. Theunbalance state can also be represented by a vector, symbolized as M inFIG. 5, the tip location of which can be plotted in polar coordinates.The Cartesian coordinates for the tip location of the unbalance statevector M may then be determined based on the number of units of theunbalance state from a vertical axis (Mx) and the number of units from ahorizontal axis (My).

Referring back to FIG. 4, for each unbalance state calculated in step104, a determination can be made whether or not the unbalance state isat least a threshold amount away from other unbalance states (step 106).The threshold amount may be referred to as a delta mass-length andsymbolized as ΔM. The threshold amount may also be viewed as a thresholdboundary of radius ΔM (see circles in FIG. 5) about a given unbalancestate (indicated by “+” symbols in FIG. 5). Other threshold criteria maybe utilized to achieve other shapes for the threshold boundary, such assquares, rectangles, etc. If an unbalance state is not at least ΔM awayfrom other unbalance states, then the unbalance state is treated asredundant and may be ignored or discarded (step 110). If the unbalancestate is at least ΔM away from other unbalance states, corresponding topreviously determined unique vibration data points, then the newvibration data point for which the latest unbalance state was derivedwill be treated as a unique vibration data point (step 108), meaningthat its unbalance state will be included in the subsequent calculation(in step 114) of a two-axis Cartesian arithmetic mean unbalance state.After a determination is made whether or not the vibration data pointbeing currently processed will be included or ignored, a determinationis made (step 112) whether the data set includes unprocessed vibrationdata points. If additional data points need to be processed, the processreturns to step 102; if not, then the mean unbalance state for the setof unique unbalance states is calculated (step 114).

FIG. 5 is a plot showing unbalance states for a plurality of vibrationdata points for a plurality of shaft speed ranges. In this example, theplotted unbalance states correspond to vibration data points acquired atshaft speeds having magnitudes within one of six shaft speed ranges S1through S6. Each unbalance state is represented in FIG. 5 by arespective encircled “+” symbol. Any other suitable graphical symbolcould be used. In FIG. 5, each unbalance state symbol, in turn, islabeled to indicate which shaft speed range includes the engine shaftspeed at the time when the corresponding unique vibration data point wasacquired. For example, FIG. 5 shows two unbalance states whosecorresponding unique vibration data points were acquired when the shaftspeed was in range S1, five unbalance states whose corresponding uniquevibration data points were acquired when the shaft speed was in rangeS4, and so forth. The intent of FIG. 5 is to depict a plurality ofunique unbalance states in the upper right-hand quadrant and a pair 60of unbalance states which are not unique with respect to each other inthe upper left-hand quadrant. Because each of the unbalance states ofpair 60 lie within the threshold range ΔM of the other, the data pointwhich is derived second will be discarded while the first data point isretained and treated as a unique unbalance state.

As shown in the plot of FIG. 5, each unbalance state can be specified byrespective Mx and My coordinates, or a radius and angle corresponding tothe vector representation for the respective unbalance state. The meanunbalance state vector M can then be determined by computing thearithmetic mean of the Mx coordinates and independently computing thearithmetic mean of the My coordinates, the resulting mean Mx and mean Mycoordinates specifying the mean unbalance state 58, indicated by an X.

The length of the mean unbalance state vector M from the origin to themean unbalance state 58 corresponds to the magnitude (i.e., mass timesdisplacement) of the balancing needed, while the angle α of vector Mrelative to the Mx axis represents the phase angle of the mean unbalancestate vector. As previously noted, the balance solution can be achievedin different ways, for example, by attaching a mass m displaced by adistance D along a vector having a phase angle (α—180°) or by attachinga mass 2 m displaced by a distance D/2 along a vector having a phaseangle (α—180°) and so forth. In accordance with one embodiment, thecircumferential locations of the balancing masses are equidistant from acenterline of the engine shaft. If the balancing mass attachment pointsare disposed along a circle of radius R, then the balancing mass forthis example (assuming that one of the attachments is at angle (α—180°))would be nm, where n=D/R. Alternatively, the same could be accomplishedby combining the effects of two or more balancing masses attached atrespective angles.

FIG. 6 shows additional details of the balancing method described above,which method is not limited to use in balancing aircraft engines, butrather has application in balancing other types of rotating systems. Themethod uses various input parameters 38, including shaft speed data froma tachometer (or other shaft speed sensor), vibration data fromaccelerometers, and time data for correlating vibration data and shaftspeed data. The phase of a vibration can be determined, for example, bycomparing the peak amplitude of the vibration to the pulses output bythe shaft speed sensor (which may output one pulse per revolution). Thebalancing process shown in FIG. 6 can then be performed for each planeof imbalance that is accessible. One or more accelerometers can be usedto acquire vibration data for each plane of imbalance.

Block 40 in FIG. 6 represents the physical step of operating therotating system through a full shaft speed range and other systemdynamic characteristic variables, including thermal stability and othertime- and load-dependent variable ranges comprising the operatingenvelope. After vibration data has been acquired, the unbalance state iscalculated (step 42) using pre-stored influence coefficients 44 that arecharacteristic of engines of the model or type being balanced. Onlysufficiently large changes in unbalance state are treated as beingunique to ensure that time dwells under similar variable conditions donot adversely influence the mean unbalance state calculation. The meanunbalance state is then determined (step 46) by calculating a two-axisCartesian arithmetic mean (as previously described with reference toFIG. 5). The analytical steps (i.e., steps 42 and 46 seen in FIG. 6) canbe performed on the ground after the aircraft has landed or onboard theaircraft during the flight.

After the mean unbalance state has been calculated, the vibration dataprocessor outputs a proposed balance solution that has the samemagnitude as the mean unbalance state but is shifted 180 degrees inphase to counteract imbalance points recorded during flight, accountingfor the relationships of shaft speed (i.e., N1) and additionaloperational parameters to vibration. Then based on this balancesolution, the physical step 48 of attaching one or more balancing massesat respective attachment points on the rotating component is performed.In one example depicted in FIG. 7, the resultant balancing solution isequal in magnitude to the mean of the unique unbalance states and phaseshifted 180 degrees. Optionally, the balance solution can be validatedby flying the airplanes with balanced engines (step 50 in FIG. 6). Themagnitude of the accelerations during re-flight may be significantlylower.

In accordance with an alternative embodiment, a system for processingdata from one or more vibration sensors that detect vibrations producedby an engine during flight is provided onboard the aircraft. This systemcomprises a computer system, programmed to perform of the operationsdepicted in FIG. 4. This system may take the form of an AVM box or DFDRthat incorporates a vibration data analyzer. This would enable thevibration data analysis to be performed onboard the aircraft duringflight.

FIG. 8 shows components of one embodiment of a system for monitoring andanalyzing engine vibrations during flight and then downloading thebalance solution to maintenance personnel. For the sake of illustration,the monitoring and analysis of vibrations produced by a turbofan engine12 on an aircraft will be described. At least one vibration sensor 2 isattached to a non-rotating structure of the engine to detect vibrationsproduced by rotating engine components. An AVM box 4, incorporating acomputer system programmed to perform the steps depicted in FIG. 4,receives the output from vibration sensor 2 during the flight.

It is known to provide an aircraft with a central maintenance computerfunction (CMCF). The CMCF encompasses major avionics, electrical, andmechanical systems installed on the aircraft. The CMCF collects, stores,and displays maintenance information generated by line commandableunits. The CMCF also provides a centralized location to initiate systemtests. The CMCF has operator interface display and input devices (i.e.multi-purpose control display units (MCDU)). In the embodiment depictedin FIG. 8, the output of the AVM box 4 is stored in a locationaccessible to a CMCF 6.

The prior art provides airline mechanics with an electronic maintenanceterminal display that displays real-time CMCF data screens via MCDUemulation. A maintenance terminal 8 is typically a laptop PC comprisinga cursor control device, a keyboard, an internal hard drive, a floppydiskette drive, a CD-ROM drive, and a graphical output printer bus.Using such a maintenance terminal, authorized personnel are able toaccess maintenance applications that supervise the aircraft's healthstatus. The onboard network of the airplane is accessible frommaintenance terminal 8 via either a wireline or wireless communicationpathway. In the embodiment shown in FIG. 8, the output of the AVM box 4can be retrieved by the CMCF 6 and downloaded to the maintenanceterminal 8, where it can be viewed on a display screen.

The balancing method disclosed herein can be used to determine where andhow many balancing masses should be added to an engine or can be used todetermine how existing balancing masses are to be adjusted, for example,by adding mass, by moving one or more attached masses to differentlocations, or by removing one or more attached masses and substitutingone or more different masses at the same or different locations.

The above-described balancing methodology may reduce if not minimizeengine vibrations. In the case of an airplane, this reduction in enginevibration results in decreased transmitted cabin noise and vibrationlevels along with decreased (cyclical) stress in the support structures.Thus, this balancing method provides an aircraft which operates morequietly and which is subject to less fatigue. Therefore, soundinsulation and structural weight may be reduced. The disclosed balancingmethod also eliminates the cost associated with re-working aircraftengines and verification re-flights.

While various embodiments have been described, it will be understood bythose skilled in the art that various changes may be made andequivalents may be substituted for elements thereof without departingfrom the scope of the teachings herein. In addition, many modificationsmay be made to adapt a particular situation to those teachings withoutdeparting from the scope thereof. Therefore it is intended that scope ofthe claims set forth hereinafter not be limited to the disclosedembodiments.

As used in the claims, the term “computer system” should be construedbroadly to encompass a system having at least one computer or processor,and which may have two or more interconnected computers or processors.

Furthermore, the method claims set forth hereinafter should not beconstrued to require that the steps recited therein be performed inalphabetical order or in the order in which they are recited.

The invention claimed is:
 1. A system comprising: an engine comprising arotating system; a plurality of balancing masses attached to therotating system, which plurality includes a first subset of balancingmasses attached to the rotating system at a time prior to calculation ofa mean unbalance state of the first subset and a second subset ofbalancing masses attached to the rotating system at a time subsequent tocalculation of a mean unbalance state of the first subset; a pluralityof vibration sensors attached to the engine; and a computer systemprogrammed to perform the following operations: (a) acquiring vibrationsensor data from the vibration sensors during rotation of the rotatingsystem while the first subset of balancing masses are attached to therotating system and the second subset of balancing masses are notattached to the rotating system; (b) converting the vibration sensordata output from the vibration sensor during operation (a) intovibration data points, the vibration data points comprising amplitudeand phase data; (c) calculating respective unbalance states for thevibration data points resulting from operation (b); (d) identifyingunbalance states calculated in operation (c) which differ from eachother by at least a threshold amount; and (e) calculating a meanunbalance state having a magnitude and an angle using unbalance statesidentified in operation (d), wherein the second subset of the pluralityof balancing masses have respective masses and locations which, whentreated as respective vectors originating at an axis of rotation andsummed, have a vector sum equal to a mass-length equivalent of the meanunbalance state calculated during operation (e) at a phase angle 180degrees from the angle associated with the mean unbalance state.
 2. Thesystem as recited in claim 1, further comprising memory having a set ofinfluence coefficients that are characteristic of a model of the enginestored therein, wherein the computer system is further programmed toperform the following operation: deriving influence coefficients fromthe set of influence coefficients, wherein operation (c) comprisesvector multiplication of vibration data points times the derivedinfluence coefficients.
 3. The system as recited in claim 2, wherein thederived influence coefficients are a function of at least a shaft speedof the engine at the time when the respective vibration data point wasacquired.
 4. The system as recited in claim 1, wherein the rotatingsystem is a gas turbine engine.
 5. The system as recited in claim 4,wherein the gas turbine engine is mounted to an aircraft.
 6. The systemas recited in claim 1, wherein the rotating system comprises an internalturbofan engine component having an outer periphery with a plurality ofthreaded holes located around the outer periphery, and the plurality ofbalancing masses comprise respective balancing screws threadablyinserted into respective threaded holes of the plurality of threadedholes.
 7. The system as recited in claim 1, wherein the balancing masseshave different masses.
 8. A system comprising: an engine comprising arotating system; a plurality of balancing masses attached to therotating system; a plurality of vibration sensors attached to theengine; and a computer system programmed to perform the followingoperations: (a) acquiring vibration sensor data from the vibrationsensors during rotation of the rotating system while none of theplurality of balancing masses are attached to the rotating system; (b)converting the vibration sensor data output from the vibration sensorduring operation (a) into vibration data points, the vibration datapoints comprising amplitude and phase data; (c) calculating respectiveunbalance states for the vibration data points resulting from operation(b); (d) identifying unbalance states calculated in operation (c) whichdiffer from each other by at least a threshold amount; and (e)calculating a mean unbalance state having a magnitude and an angle usingunbalance states identified in operation (d), wherein the plurality ofbalancing masses have respective masses and locations which, whentreated as respective vectors originating at an axis of rotation andsummed, have a vector sum equal to a mass-length equivalent of the meanunbalance state calculated during operation (e) at a phase angle 180degrees from the angle associated with the mean unbalance state.
 9. Thesystem as recited in claim 8, further comprising memory having a set ofinfluence coefficients that are characteristic of a model of the enginestored therein, wherein the computer system is further programmed toperform the following operation: deriving influence coefficients fromthe set of influence coefficients, wherein operation (c) comprisesvector multiplication of vibration data points times the derivedinfluence coefficients.
 10. The system as recited in claim 9, whereinthe derived influence coefficients are a function of at least a shaftspeed of the engine at the time when the respective vibration data pointwas acquired.
 11. The system as recited in claim 8, wherein the rotatingsystem is a gas turbine engine.
 12. The system as recited in claim 11,wherein the gas turbine engine is mounted to an aircraft.
 13. The systemas recited in claim 8, wherein the rotating system comprises an internalturbofan engine component having an outer periphery with a plurality ofthreaded holes located around the outer periphery, and the plurality ofbalancing masses comprise respective balancing screws threadablyinserted into respective threaded holes of the plurality of threadedholes.
 14. The system as recited in claim 8, wherein the balancingmasses have different masses.